1. Field of the Invention
This invention relates generally to monitoring and control of corrosion of oilfield wells and oil and gas pipelines, and, more specifically, to the use of time-domain-reflectometry methods for in-situ monitoring of such corrosion.
2. Background of the Art
Wellbores or wells are formed in subsurface formations to recover hydrocarbons trapped in such formations. Such wells are usually lined with a metal (typically steel) liner or casing. Cement is packed in the space (annulus) between the wellbore and the casing. Perforations made in the casing allow the flow of the hydrocarbons from the formations to the casing via the perforations. A tubing disposed inside the casing extending from the surface to a location above the perforations carries the formation fluid containing the hydrocarbons to the surface. A metallic screen is placed between the casing and the tubing to prevent the flow of solids from the formation to the tubing. A number of metallic devices, including fluid flow control valves, safety valves, etc. are disposed in the casing and/or the tubing to facilitate the flow of the formation fluids to the surface. The formation fluid is generally under relatively high pressure (sometimes greater than 10,000 psi) and at relatively high temperature (often above 150.degree. F.). Frequently, formation fluid flows into the wellbore at a relatively high velocity.
The formation fluid often includes corrosive elements, such as salt water and sulfates. Such elements corrode the metallic tubular members and other devices in the wellbores. Corrosion accelerates at high temperatures and pressures. Corrosion inhibits the production of the hydrocarbons and, if not controlled, can destroy the equipment in the well. This can lead to extensive reworking of the well or, in extreme cases, abandoning of the well.
Formation pressure in older wells or at shallow production zones is often not sufficient to cause the formation fluid to rise to the surface. In such wells, electrical submersible pumps are commonly utilized to recover the formation fluid from such wells. Corrosive fluids in the well can be very detrimental to the operation of such relatively expensive equipment.
Oil and gas pipelines carry large amounts of hydrocarbons from the oil and gas fields to the processing plants over great distances. Pump stations along the pipelines move the hydrocarbons through the pipelines. Such hydrocarbons often carry chemicals which corrode the pipelines and the equipment in the pipelines.
A variety of chemicals (also referred to herein as the "additives") are often selectively injected into the fluid flowing through the wells or the pipelines (collectively referred to herein as hydrocarbon-carrying conduits) to inhibit the corrosion. To monitor the corrosion, various devices are periodically inserted into the hydrocarbon-carrying conduits to determine the extent of the corrosion. In one method, a sacrificial corrosion measuring device is conveyed into the wellbore to a selected depth. The device is retrieved after a known time period and analyzed. The corrosion in the wellbore is estimated from the corrosion of the sacrificial device. In another method, samples of the wellbore fluid are taken downhole and analyzed to estimate the corrosive effects on the liner and other devices in the wellbore. In another method, eddy current devices carried by wireline tools are run though the well to determine the pits, cracks and voids in the well liner.
The above-noted and other prior art methods usually require periodic trips to the well site or to the pumping station for the deployment of the corrosion monitoring tools in the wellbore or pipeline to measure corrosion effects. The test results are analyzed to determine the corrective actions, which may include remedial actions, such as workover, replacement or repair of equipment, addition of chemicals into the wellbore or the pipeline, etc. Such corrosion-measuring methods are relatively expensive and do not provide in-situ measurements for quick action.
The present invention provides a system and method for periodic and/or continuous in-situ monitoring of corrosion in wellbores and pipelines. The system can automatically adjust the amounts of chemicals introduced into the wellbores or pipelines based on the in-situ measurements.